Although I have no connection to the company, last year I consulted on some horticultural aspects for a client in California that had a brand new Texas Greenhouse structure. Seemed to be well made and designed.

http://www.houzz.com/photos/query/greenhouse-architect.

Those are all general architects. Need someone with real greenhouse experience.

If you want high end, take a look at this: Clayton Hill Greenhouses

What do you need an architect to do that could not be done by the people who build and supply the type of building you want? I'd start by finding a product line you find desirable and find out what services are available from the company. Most, especially those that do high end custom building, provide a range of design and support services.

BC Greenhouse's custom or Meridian Estate options.

I designed mine myself. If I can, so can you. It was, however, a significant investment in time. http://forums.gardenweb.com/discussions/4632289/m=23/green-house-completed